I think the issue here is that the article is talking about two things: a common resource management idiom, and the cool fact that `under(f, g) = f^1 * g * f`

I think the issue here is that the article is talking about two things: a common resource management idiom, and the cool fact that `under(f, g) = f^1 * g * f` implements it. `with` implements this idiom just fine, too, but `under` is a more general solution that can solve even more problems, if you're willing to be a bit more careful about `f` and `g`. In the Read/Open case, for example, it looks like Read must return both the read result and the file handle, and Open^-1 must take the read result as input and forward it back out as output. Feasible, but unintuitive for the implementers. If we were actually implementing this resource management idiom, I'm not sure we would use `under`, even though we could. If we want to write `f`, `f^-1`, and `g` with the more natural `with` semantics, it's clearer to define our own metafunction from scratch: `with(f, g)(x) = (g(f_result), f^-1(f_result)) where f_result = f(x)`. edit: The follow-up post actually touches on how Read/Open really isn't a great example of `under`. Interesting. http://prog21.dadgum.com/122.html http://prog21.dadgum.com/122.html